A 32-year-old primigravida at 28 weeks of gestation presents to the antenatal clinic with painless vaginal bleeding. On examination, vital signs are stable and the uterus is soft and non-tender. No vaginal examination is performed. What is the investigation of choice to confirm the diagnosis of placenta previa?

Explanation

## Investigation of Choice for Placenta Previa **Key Point:** Transvaginal ultrasound (TVS) is the **gold standard** investigation for confirming the diagnosis of placenta previa, offering superior accuracy over transabdominal ultrasound in localizing the placenta relative to the internal cervical os. ### Why Transvaginal Ultrasound? 1. **Superior Diagnostic Accuracy** - TVS has sensitivity and specificity approaching **98–100%** for placenta previa - Provides precise measurement of the distance between the placental edge and the internal cervical os - Overcomes limitations of TAS such as maternal obesity, posterior placenta, and bladder filling artifacts 2. **Safety Profile** - Contrary to intuition, TVS is **safe** in suspected placenta previa — the probe is placed in the vagina but does not reach the cervical os - Multiple studies (including RCOG guidelines) confirm TVS does not precipitate hemorrhage - Recommended by RCOG, ACOG, and ISUOG as the preferred modality 3. **Limitations of Transabdominal Ultrasound (TAS)** - TAS is the **first-line screening tool** but has a false-positive rate of up to 25–50% (especially for posterior placenta and low-lying placenta) - Requires a full bladder, which can distort anatomy and overdiagnose previa - When TAS is inconclusive or suggests low-lying placenta, TVS is mandatory for confirmation 4. **Role of MRI** - MRI is reserved for cases where placenta accreta spectrum is suspected, not for routine diagnosis of placenta previa **High-Yield:** TAS is the **initial/screening** investigation; TVS is the **confirmatory gold standard**. This distinction is critical for exam purposes (RCOG Green-top Guideline No. 27; Williams Obstetrics, 25th ed.). ### Comparison of Modalities | Modality | Role | Accuracy | | --- | --- | --- | | **Transabdominal US** | First-line screening | ~75–90% (false positives common) | | **Transvaginal US** | Gold standard confirmation | ~98–100% | | **MRI** | Accreta spectrum evaluation | High, but not first-line | | **Digital exam** | **Contraindicated** | Risk of massive hemorrhage | **Clinical Pearl:** Digital cervical examination is absolutely contraindicated in suspected placenta previa as it can precipitate life-threatening hemorrhage. Speculum examination may be used to assess the source of bleeding but does not confirm placenta previa. TVS is the definitive investigation of choice. **Reference:** Williams Obstetrics, 25th edition; RCOG Green-top Guideline No. 27 (Placenta Praevia and Placenta Accreta).